The present invention relates to methods, systems and apparatuses for measuring a magnetic field, more particular to such methods, systems and apparatuses for measuring the distribution of magnetic field over, through or along an extended distance, area or volume in space.
There are different types of conventional sensors, based on various principles of physics, which measure the magnetic field at a specific point in space. One type of conventional sensor (commonly known as a xe2x80x9cfluxgate magnetometerxe2x80x9d) takes advantage of the magnetic properties of ferromagnetic cores. Another type of conventional sensor takes advantage of magneto-resistive properties of materials.
These and possibly other types of conventional sensors not only are more typically utilized to measure a magnetic field at a specific point in space, but can also be configured to integrate the magnetic field along an extended distance in space. However, in order to measure the distribution of magnetization and/or magnetic field intensity over a spatial distance, area or volume, several conventional sensors must be distributed throughout the space of interest. Therefore, many sensing elements and their individual electronics are conventionally required. Such approaches involving sensor array(s) are disadvantageous in terms of efficiency, cost and reliability.
The fiber optic sensor (also abbreviatedly referred to as the xe2x80x9coptic sensorxe2x80x9d) does not measure the magnetic field at a point; rather, this type of conventional sensor measures the average magnetic field over the length of the optic sensor. The optic sensor takes advantage of certain optical properties of glass fibers to detect and quantify the magnetic field over a limited distance, namely, the length associated therewith.
Still, as regards fiber optic sensing, in order to integrate the magnetic field along an extended distance in space (i.e., measure the distribution of magnetization and/or magnetic field intensity over a spatial distance, area or volume), the conventional approach remains sensor arrayal, according to which a plurality of optic sensors are arranged within the space of interest.
Pertinent background information is provided by the following papers, each of which is hereby incorporated herein by reference:
Lenz, J. E., xe2x80x9cA Review of Magnetic Sensors,xe2x80x9d IEEE Proceedings, Vol. 78, No. 6, Jun. 1990; Gordon, D. I., R. E. Brown and J. F. Haben, xe2x80x9cMethods for Measuring the Magnetic Field,xe2x80x9d IEEE Trans. Mag., Vol. Mag-8, No. 1, March 1972; Gordon, D. I. and R. E. Brown, xe2x80x9cRecent Advances in Fluxgate Magnetometry,xe2x80x9d IEEE Trans. Mag., Vol. Mag-8, No. 1, March 1972.
In view of the foregoing, it is an object of the present invention to provide method, apparatus and system for measuring magnetic field distribution in a sector of interestxe2x80x94e.g., a distance, an area or a volumexe2x80x94which includes a plurality of different spatial points (e.g., distinct, discrete or noncoincident locations in space).
It is another object of the present invention to provide such method, apparatus and system which are more efficient than conventional methods, apparatuses and systems.
It is a further object of this invention to provide such method, apparatus and system which are more economical than conventional methods, apparatuses and systems.
It is another object of this invention to provide such method, apparatus and system which are more reliable than conventional methods, apparatuses and systems.
In accordance with many embodiments of the present invention, a system is provided for determining the distribution of a magnetic field in a spatial sector. The inventive system comprises: means for measuring a magnetic field amplitude value at each of a plurality of points in the sector, wherein the means for measuring is characterized by a length which is defined by the points; means for applying alternating current at a high frequency having an associated wavelength which corresponds to a multiple of the length; means for conducting the applied alternating current so as to establish a standing wave along the length; and, means for processing the measured magnetic field amplitude values, the means for processing including means for performing, over the multiple of the length, Fourier analysis based on a harmonic bias function which results from the standing wave.
According to many inventive embodiments, a method is provided for determining the distribution of a magnetic field in a spatial sector. The inventive method comprises: measuring a magnetic field amplitude value at each of a plurality of points in the sector, wherein the measuring is characterized by a length which is defined by the points; applying alternating current at a high frequency having an associated wavelength which corresponds to a multiple of the length; conducting the applied alternating current so as to establish a standing wave along the length; and, processing the measured magnetic field amplitude values, the processing including performing, over said multiple of said length, Fourier analysis based on a harmonic bias function which results from the standing wave.
According to this invention, a magnetic field amplitude value is determined at one point (location), more typically at each of at least two points (locations). In generally preferred inventive practice, the magnetic field amplitude value can be determined at each of any plural number of points which define the characteristic length of the measuring means. Indeed, such length can be defined by an unlimited (at least two and potentially infinite) number of such points. Moreover, the possible configurations of such points are unlimited in terms of their relative positions and distances. The considered points can be arranged, with respect to each other, in any combination of connectedness (one or more continuities) and/or disconnectedness (one or more discontinuities).
The present invention features the utilization of a sinusoidal standing wave bias field in order to directly measure, in a spatially continuous way, the Fourier coefficients of a distribution of magnetic field. A main advantage of the present invention is that it requires only one set of electronics. In principle, a single inventive sensor can measure the same magnetic field distribution as can a conventional array of sensors having thousands of elements. Moreover, many inventive embodiments can be practiced utilizing various commercially available, off-the-shelf components.
A solenoid is a current-carrying coil (e.g., cylindrical coil), typically of insulated wire, in which an axial magnetic field is established by a flow of electric current. According to a conventional magnetic field sensor, a solenoidal winding is placed around the sensor and powered with a dc current. The solenoidal winding can apply a bias field to the sensor. The purpose of the bias field is to cancel out the earth""s magnetic field so as to improve the sensor""s linearity over its dynamic range, and to apply a calibration field to the sensing element.
By contrast, the inventive magnetic field sensor uses a bias field which is generated by a radio frequency alternating current source, thereby establishing a standing wave bias. By inventively varying the wavelength of a simple harmonic (e.g., sinusoidal or cosinusoidal) bias field over an integrating sensor, the spatial Fourier Coefficients over the length of the magnetometer can be measured.
According to many conventional control systems, the spatial Fourier coefficients of the distribution of magnetic field are required measured quantities. The Fourier coefficients can be obtained by numerically computing them with magnetic field measurements at discrete locations using an array of conventional sensors. Each of the sensor elements in the array requires its own electronics and data acquisition channel. If the array is large or if the application requires high harmonic content, then the measurement array could be quite costly and may have problems with reliability.
By contrast, the inventive xe2x80x9cstanding wave magnetometerxe2x80x9d can directly measure the spatial Fourier coefficients up to very high harmonic values with only one set of electronics and one data acquisition channel. In addition, the measurement is continuous over the length of the invention""s extended integrating sensing element, which avoids spatial sampling issues encountered by a conventional array of sensors that measure the magnetic field at discrete locations in space.
According to conventional methodologies, applications wherein the distribution of a magnetic field is the desired measurement could require a very large array of discrete sensors. This very large array would depend upon the spatial extent of the array or on the required fidelity of the system.
By contrast, according to the present invention, the inventive standing wave magnetometer measures the spatial Fourier coefficients. According to many inventive embodiments, the magnetic field along the associated length which includes the considered spatial points can then be mathematically computed using the inverse Fourier sine (or cosine) series (or transform). According to some inventive embodiments, the Fourier coefficients themselves (which can be obtained directly by practicing the present invention) are the desired measurement parameter.
The inventive magnetometric methodology thus determines the distribution of a magnetic field by using only one electronic package and one data acquisition system. If a conventional array is large or if an application requires high spatial fidelity, then the inventive standing wave magnetometer could significantly reduce the cost of the system and increase its reliability.
The present invention is potentially useful for a variety of applications, both military and commercial. The invention could be used inboard U.S. Naval vessels in association with an xe2x80x9cAdvanced Closed-Loop Degaussing System.xe2x80x9d
In addition, the invention could be implemented in association with an underwater-based (e.g., as part of an underwater submarine barrier array, in naval mines, etc.) or land-based intruder-detection system.
Furthermore, the invention has commercial applicability in different geological and geophysical contexts (e.g, in geophysical prospecting for minerals, in geophysical studies, etc.).
Another potential commercial application of the present invention is in the realm of traffic control (e.g., multi-lane vehicle detection).
Other objects, advantages and features of this invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.